1. Field of the Invention
This invention relates to apparatus for and a method of monitoring a number of geophysical and other physical variables by way of the refractive index or, indirectly, the dielectric constant of materials. Specifically, it related to monitoring of such properties as soil and pavement moisture content; the moisture content of bulk food products such as grains and beans; liquid levels in storage tanks; interface levels between water and floating layers of oil or other petroleum products; the thickness of ice layers; the frozen/unfrozen interface in partially frozen ground; the location of liquid and gas leaks on roofs, in landfill liners, geosynthetic membranes, and pipelines; and the location of electrical short circuits, open circuits, and impedance discontinuities in cables. The detection technique is based on the reflection of frequency-modulated continuous waves during their propagation along transmission line probes embedded in the material being tested.
2. Prior Art
The frequency-modulated continuous wave technique has been used in radar system applications wherein electromagnetic waves are transmitted into space from antennas. Applications employing transmission lines have been disclosed in the inventor's previous application directed to the detection of water-sediment interfaces around bridge piers and pilings (Ser. No. 08965483, filed Nov. 6, 1997) and another application relating to the detection of frazil ice on underwater gratings (Ser. No. 08871624, filed Jun. 9, 1997).